This invention relates to a process of addition polymerization, especially olefin polymerization, and to a catalyst system therefor. In particular the invention relates to a multistage polymerization process effected using a multi-site, preferably dual-site polymerization catalyst.
The molecular weight distribution (MWD) of a polymer affects the properties of the polymer, in particular its mechanical strength and processability. Long term mechanical strength to a large extent is determined by the high molecular weight fraction and extrudability to a large extent is determined by the low molecular weight fraction. The mechanical strength moreover can be manipulated by the inclusion of xcex1-olefin comonomers, with it thus being possible to vary the nature and relative content of the side chains so introduced. This is particularly important for the high molecular weight portion of a broad MWD polymer, e.g. a PE polymer, and thus the comonomer content of the high molecular weight portion may typically be desired to be greater than that in the low molecular weight portion which latter may be a homopolymer. Accordingly polymers with a broad or multimodal (e.g. bimodal) MWD find many uses as for example in blow moulding, films, pipes, wire, cable, etc., where a combination of strength and extrudability is particularly important.
Certain olefin polymerization catalysts are generally less suitable for the single stage preparation of polymers for such uses because the MWD for the polymers they produce is too narrow and as a result the polymer may be difficult to process.
The preparation of broad MWD olefin polymers is described for example in EP-A-310734, EP-A-128045 and NO-923334.
Thus broad MWD olefins can be made in a dual reactor system (e.g. as described in NO-923334) using a variety of transition metal catalysts, e.g. Ziegler catalysts. The broad MWD results in this case from the process conditions in the different reactors favouring the production of different molecular weight polymers, e.g. one favouring the production of a higher molecular weight polymer and another favouring production of a lower molecular weight polymer.
Some catalysts in a single polymerisation stage, for example chromium oxide catalyst, inherently give broad MWD in a single polymerisation stage, however such catalysts in general tend to include alpha olefin comonomers in the low MW part, resulting in poorer mechanical properties.
Ideally, to achieve the most advantageous polymer properties in a multi-stage bimodal polymerisation process, an optimum polymer product would contain a very small amount of very high molecular weight polymer and a large amount of very low molecular weight polymer. However, such a polymer product has proved difficult to synthesize due to the formation of polymeric gels. These gels originate due to the difficulty of meltmixing particles of very different melt viscosities. The cause of the polymer particles arriving at the extrusion step having widely different melt viscosities is the broad residence time distributions of the polymerisation stages.
It is often a requirement that the melt strength of the polymer product is high (for example for blow moulding). However, though this requirement has been met in single stage polymerization using chromium oxide catalysts to give monomodal polymers, to achieve a high melt strength polymer from a staged polymerization, especially making a bimodal polymer, is still a problem.
We have now found that the MWD of a polyolefin can be particularly effectively tailored to suit the needs of the user of the polyolefin, e.g. the producer of blow moulded objects, films, cables, tubes and pipes, etc., if polymerization is effected in at least two reaction stages using a catalyst material, generally a particulate material, that contains at least two different types of active polymerization sites wherein one of the sites gives rise to polymers of differing molecular weights depending on the concentration of hydrogen in the reactor and the other site gives rise to polymers of substantially the same molecular weight irrespective of the hydrogen concentration. Typically such a catalyst material may contain a particulate multi-site component, optionally together with, in a liquid phase or not, co-catalysts and adjuvants.